U.S. patent application number 11/760295 was filed with the patent office on 2008-12-11 for method and system for adjusting an optical model.
This patent application is currently assigned to QIMONDA AG. Invention is credited to Stefan Blawid, Sebastian Champigny, Wolfram Kostler, Rainer Pforr, Jens Reichelt, Manuel Vorwerk, Thorsten Winkler, Ralf Ziebold.
Application Number | 20080304029 11/760295 |
Document ID | / |
Family ID | 40095571 |
Filed Date | 2008-12-11 |
United States Patent
Application |
20080304029 |
Kind Code |
A1 |
Pforr; Rainer ; et
al. |
December 11, 2008 |
Method and System for Adjusting an Optical Model
Abstract
In a method of adjusting an optical parameter of an exposure
apparatus, a photolithographic projection is performed using an
exposure apparatus and using a layout pattern so as to provide
measured layout data with different focus settings of the exposure
apparatus. An optical model is provided including at least one
optical parameter and a simulated image is created by using the
optical model and the layout pattern. The optical model is
optimized by modifying the optical parameter.
Inventors: |
Pforr; Rainer; (Dresden,
Weixdorf, DE) ; Winkler; Thorsten; (Unterhaching,
DE) ; Ziebold; Ralf; (Radebeul, DE) ; Kostler;
Wolfram; (Langbrueck, DE) ; Reichelt; Jens;
(Dresden, DE) ; Blawid; Stefan; (Dresden, DE)
; Champigny; Sebastian; (Munchen, DE) ; Vorwerk;
Manuel; (Dresden, DE) |
Correspondence
Address: |
EDELL, SHAPIRO & FINNAN, LLC
1901 RESEARCH BLVD., SUITE 400
ROCKVILLE
MD
20850
US
|
Assignee: |
QIMONDA AG
Munich
DE
|
Family ID: |
40095571 |
Appl. No.: |
11/760295 |
Filed: |
June 8, 2007 |
Current U.S.
Class: |
355/44 |
Current CPC
Class: |
G03F 1/36 20130101; G03F
7/705 20130101 |
Class at
Publication: |
355/44 |
International
Class: |
G03B 27/14 20060101
G03B027/14 |
Claims
1. A method of adjusting an optical parameter of an exposure
apparatus, comprising: performing a photolithographic projection
using an exposure apparatus and using a layout pattern so as to
provide measured pattern data as printed on a substrate with
different focus settings of the exposure apparatus; providing an
optical model to describe the exposure apparatus, the optical model
including at least one optical parameter; creating a simulated
image by using the optical model and the layout pattern, the
simulated image being calculated with different focus settings of
the exposure apparatus; and optimizing the optical model by
adjusting the at least one optical parameter so as to reduce an
overall difference between the measured pattern data and the
simulated image for the different focus settings, the overall
difference being determined along a first direction and a second
direction in the measured pattern data and in the simulated
image.
2. The method according to claim 1, wherein minimizing differences
of focus conditions further comprises: calculating error values
between the measured pattern data and the simulate image along the
first and second directions for different focus settings; and
adjusting the optical parameter so as to minimize the error
values.
3. The method according to claim 1, wherein the optical parameter
comprises an aberration parameter.
4. The method according to claim 3, wherein an initial value of the
aberration parameter is derived by performing a wavefront
measurement of a projection system of the projection apparatus.
5. The method according to claim 3, wherein the aberration
parameter is described as a Zernike polynomial having
coefficients.
6. The method according to claim 5, wherein the coefficients are
modified during optimization of the optical model.
7. The method according to claim 3, wherein the aberration
parameter is measured in a steady state of a projection system.
8. The method according to claim 7, wherein the steady state
includes the steady state of thermal heating of one or more lens
elements of the exposure apparatus.
9. The method according to claim 1, wherein the optical parameter
is adjusted so as to minimize a best focus difference, the best
focus difference being determined as a minimum of the overall
difference.
10. The method according to claim 1, wherein the first and second
directions are substantially perpendicular to each other.
11. A method of simulating lithographic projection, comprising:
providing at least one parameter adapted to describe aberration of
a projection system including an illumination source suitable to
emit polarized light; providing layout data and generating a
reticle from the layout data; performing a photolithographic
projection to create a pattern using the illumination source and
the reticle and measuring pattern data from the pattern created for
different focus settings; providing an optical model including the
at least one parameter; creating a simulated image by using the
optical model and the layout data for different focus settings;
comparing the measured pattern data and the simulated image; and
optimizing the at least one parameter by reducing an overall
difference between the measured pattern data and the simulated
image for different focus settings.
12. The method according to claim 1, further comprising: using the
optical model and the optimized parameter to calculate a further
set of layout data.
13. The method according to claim 11, wherein optimization of the
at least one parameter further comprises a focus difference
calculation along a first direction and a second direction in an
image plane.
14. The method according to claim 13, wherein optimization of the
at least one parameter further comprises determining a best focus
in an image plane along the first direction and determining a best
focus in an image plane along the second direction using the
optical model.
15. The method according to claim 11, wherein the optical parameter
comprises an aberration parameter.
16. The method according to claim 15, wherein an initial value of
the aberration parameter is derived by performing a wavefront
measurement of the projection system.
17. The method according to claim 15, wherein the aberration
parameter is described as a Zernike polynomial having
coefficients.
18. A method of performing an optical proximity correction,
comprising: performing a photolithographic projection using an
exposure apparatus and using a layout pattern so as to provide
measured layout data with different focus settings of the exposure
apparatus; providing an optical model to describe the exposure
apparatus, the optical model including at least one optical
parameter; creating a simulated image by using the optical model
and the layout pattern, the simulated image being calculated with
different focus settings of the exposure apparatus; optimizing the
optical model by adjusting the at least one optical parameter so as
to reduce an overall difference between the measured pattern data
and the simulated image for the different focus settings, the
overall difference being determined along a first direction and a
second direction in the measured pattern data and in the simulated
image; and using the model to perform optical proximity correction
of the layout pattern.
19. The method according to claim 18, wherein the layout pattern
includes a cell portion and a periphery portion.
20. The method according to claim 18, wherein the optical proximity
correction comprises inserting, removing, relocating, or modifying
assist features.
21. The method according to claim 18, wherein the optical proximity
correction of the layout pattern comprises relocating and modifying
features in the layout pattern.
22. The method according to claim 18, wherein the optical proximity
correction adapts modified layout data and layout data to a desired
target image.
23. A system for adjusting an optical parameter of an exposure
apparatus, comprising: a measurement device configured to determine
a set of measured pattern data as printed on a substrate; an
optical model configured to describe an exposure apparatus, the
optical model including at least one optical parameter and being
configured to create a simulated image, the simulated image being
calculated with different focus settings of the exposure apparatus;
and a processor configured to create a simulated image from the
optical model and to optimize the optical model by adjusting the at
least one optical parameter so as to reduce an overall difference
between the measured pattern data and the simulated image for the
different focus settings, the overall difference being determined
along a first direction and a second direction in the measured
pattern data and in the simulated image.
24. The system according to claim 23, wherein the processor is
further configured adapted to perform optical proximity
corrections.
25. The system according to claim 23, wherein the processor is
further configured to perform a full layout simulation.
26. A fabrication unit for processing semiconductor products
including a system for adjusting an optical parameter of an
exposure apparatus, comprising: a measurement device configured to
determine a set of measured layout data as printed on a substrate
with different focus settings of an exposure apparatus; an optical
model adapted to describe an exposure apparatus, the optical model
including at least one optical parameter and being adapted to
create a simulated image, the simulated image being calculated with
different focus settings of the exposure apparatus; and a processor
adapted to optimize the optical model by adjusting the at least one
optical parameter so as to reduce differences between the measured
layout data and the simulated image by minimizing differences of
focus conditions determined along a first direction and a second
direction in the measured layout data and calculated along the
first and the second direction in the simulate image.
27. A method of manufacturing an integrated circuit comprising at
least one layer lithographically structured using a mask, the mask
comprising a layout from a set of layout data, the set of layout
data being calculate by using an optical model and an optimized
parameter, the method comprising: providing at least one parameter
configured to describe aberration of a projection system including
an illumination source suitable to emit polarized light; providing
layout data and generating a reticle from the layout data;
performing a photolithographic projection to create a pattern using
the illumination source and the reticle and measuring pattern data
from the pattern created for different focus settings; providing an
optical model including the at least one parameter; creating a
simulated image using the optical model and the layout data for
different focus settings; comparing the measured pattern data and
the simulated image; and optimizing the at least one parameter by
reducing an overall differences between the measured pattern data
and the simulated image for different focus settings.
Description
BACKGROUND
[0001] The process of lithographic projection of light patterns
onto photo resist layers is commonly used to form structures and
features in integrated circuits. Implementation of such processes
typically involves computer simulation of expected patterns formed
by lithographic projection using certain optical parameters. It is
desirable to improve the dimensional accuracy of such simulations
of lithographic projection processes.
SUMMARY
[0002] In a method of adjusting an optical parameter of an exposure
apparatus, a photolithographic projection is performed using an
exposure apparatus and using a layout pattern so as to provide
measured layout data with different focus settings of the exposure
apparatus. An optical model is provided including at least one
optical parameter and a simulated image is created by using the
optical model and the layout pattern. The optical model is
optimized by modifying the optical parameter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] In the accompanying drawings:
[0004] FIG. 1 illustrates an optical projection system;
[0005] FIG. 2 illustrates a layout pattern in a top view;
[0006] FIG. 3 illustrates a characterization of CD-values for
different orientations;
[0007] FIG. 4A illustrates a characterization of structural feature
sizes of a pattern according to an embodiment;
[0008] FIG. 4B illustrates a characterization of structural feature
sizes of a pattern according to a further embodiment;
[0009] FIG. 5A illustrates a characterization of fit accuracy for
different focus settings according to a further embodiment;
[0010] FIG. 5B illustrates a characterization of fit accuracy for
different focus settings according to a further embodiment;
[0011] FIG. 6 illustrates a flow chart of method steps for
performing an optical simulation;
[0012] FIGS. 7A and 7B each illustrate a simulated resist pattern
in a top view with respect to a first aberration parameter;
[0013] FIGS. 8A and 8B each illustrate a simulated resist pattern
in a top view with respect to a second aberration parameter;
[0014] FIGS. 9A and 9B each illustrate a measured resist
pattern;
[0015] FIG. 10 illustrates a flow chart of method steps for
performing an optical simulation for optical proximity
correction;
[0016] FIG. 11 illustrates a flow chart of method steps for
performing an optical simulation for circuit optimization; and
[0017] FIG. 12 illustrates a system for adjusting an optical
parameter.
DETAILED DESCRIPTION
[0018] Embodiments of methods and systems for adjusting an optical
parameter are discussed in detail below. It is appreciated,
however, that the present invention provides many applicable
inventive concepts that can be embodied in a wide variety of
specific contexts. The specific embodiments discussed are merely
illustrative of specific ways and do not limit the scope of the
invention.
[0019] In the following, embodiments of the method and the system
are described with respect to improving dimensional accuracy during
simulation of lithographic projection of a layer of an integrated
circuit. The embodiments, however, might also be useful in other
respects, e.g., pattern fidelity of two-dimensional structures,
improvements in process window calculations, improvements in
identifying critical parts of a layout of a pattern, yield
enhancement techniques or layout simulation capabilities.
[0020] Furthermore, it should be noted that the embodiments are
described with respect to line-space patterns but might also be
useful in other respects including but not limited to dense
patterns, semi-dense patterns or patterns with isolated lines and
combinations between all them. Lithographic projection can also be
applied during manufacturing of different products, e.g.,
semiconductor circuits, thin film elements. Other products, e.g.,
liquid crystal panels or the like might be produced as well.
[0021] With respect to FIG. 1, a set-up of a lithographic
projection apparatus 100 is shown in a side view. It should be
appreciated that FIG. 1 merely serves as an illustration, i.e., the
individual components shown in FIG. 1 neither describe the full
functionality of a lithographic projection apparatus 100 nor are
the elements shown in true scale. Furthermore, the described
embodiment uses a projective optical system in the UV range
employing a certain demagnification. However, other lithographic
system including proximity projection, reflective projection or the
like employing various wavelengths from the visible to ultraviolet
to extreme ultraviolet range can be employed.
[0022] The projection apparatus 100 comprises a light source 104,
which is, e.g., an Excimer laser with 193 nm wavelength. An
illumination optic 106 projects the light coming from the light
source 104 through a photo mask 102 into an entrance pupil of the
projection system. The illumination optic 106 is comprised of
several lenses 108, as shown in FIG. 1, which are arranged between
the light source 104 and photo mask 102.
[0023] The photo mask 102 comprises a mask pattern 112 composed of
light absorptive or light attenuating elements. Light absorptive
elements can be provided by, e.g., chrome elements. Light
attenuating elements can be provided by, e.g., molybdenum-silicate
elements. The mask pattern is derived from a layout pattern which
can be provided by a computer aided design system, in which
structural elements of the layout pattern are generated and
stored.
[0024] The light passing the photo mask 102, i.e., not being
blocked or attenuated by the above mentioned elements, is projected
by projection lens 114 onto the surface 124 of a semiconductor
wafer 122. The pattern projected on the semiconductor wafer 122 is
usually de-magnified, e.g., scaled down by factor of 4 or 5. For
the optical characteristics of the projection apparatus 100, the
main contributions are determined by the light source 104, the
illumination optic 106, and the projection lens 114 which are
further commonly denoted as projection system.
[0025] A photo resist film layer 126 is deposited on the
semiconductor wafer 122. Onto the resist film layer 126, the mask
pattern 112 is projected. After developing the photo resist film
layer 126, a three dimensional resist pattern 128 is formed on the
surface of the semiconductor wafer 122 by removing those parts of
the photo resist film layer 126 which are exposed with an exposure
dose above the exposure dose threshold of the resist film layer 126
(or alternatively, those parts of the photo resist film layer that
are not exposed can be removed, depending on the composition of the
photo resist film and processing substances).
[0026] Before the layout pattern is fabricated in a high volume
manufacturing process, several set-up procedures can be performed
including optimizing the illumination process and implementing so
called resolution enhancement techniques (RET) which improve the
resolution capabilities of the lithographic projection
apparatus.
[0027] Currently, there are several concepts known in the art which
address the problem of increasing the resolution capabilities.
According to a first example, off-axis illumination in the
projection system of the projection apparatus together with
sub-resolution sized assist features is used. In a second example,
the concept of alternating phase shift masks is employed so as to
enhance the resolution capabilities of the projection
apparatus.
[0028] Off-axis illumination is achieved by providing an annular-,
quasar- or dipole-shaped aperture stop in a conjugated plane of the
illumination optic 106 of lithographic projection apparatus 100
thus enhancing contrast and depth of focus of densely spaced
patterns. In turn, off-axis illumination often impairs imaging of
isolated structures. In order to allow imaging of isolated
structures, sub resolution sized assist features are used which
facilitate the resolution of these structures.
[0029] In order to achieve dimensional accuracy of the mask pattern
during imaging, the sub-resolution sized assist features are
determined using a simulation model of the photolithographic
projection. In order to perform this calculation, a model for
forming an aerial image, a model of the resist exposure, and for
the photo mask is provided. The result of the simulation is
returned to the layout program so as to alter the geometric
structures before production of photo mask 102.
[0030] The simulation includes a description of the lithographic
apparatus including different kind of optical parameters. These
parameters include but are not limited to a polarization state of
the light source 104, aberration parameters derived for the optical
projection apparatus 100, and illumination mode as achieved by the
aperture stop.
[0031] During set up of optical lithography processes, a simulation
can be performed in which desired layout patterns and simulated
images on the wafer are compared. According to this procedure,
differences between the desired layout pattern and the resist
pattern 128 can be minimized.
[0032] As an example, a fraction of a layout pattern for a specific
layer is shown in FIG. 2 in a top view. The layout pattern 200
includes a critical structure in DRAM manufacturing with a
line-space array 202 having horizontal parallel lines 204 and a
vertical line 206. The exemplary layout pattern can be used as
pattern for a layer serving as an interface between a cell array
within a memory chip and peripheral circuits. Topological
representations of memory cells and layout arrangements for
peripheral circuits are known in the art and are therefore not
discussed further.
[0033] Referring now to FIG. 3, results of a lithographic
simulation is described when using a presently available state of
the art simulation tool. Simulation tools are provided by many
manufactures, including Mentor Graphics Inc., ASML Inc., or other
companies.
[0034] In particular, the lithographic simulation includes optical
parameters for which a polarization in the TE-mode and dipole
illumination of light source 104 is chosen. The optical simulation
is performed such that, in the simulated image, the simulated line
width of the resulting structure is plotted against the beam focus
along the projection plane behind the projection optics. The
simulation is independently conducted for structures having
substantially horizontal and vertical structures, i.e., for
structures being arranged substantially perpendicular. The nominal
values of the desired layout pattern are 200 nm for both cases.
[0035] In FIG. 3, a curve 302 describing the line width versus
focus behavior for horizontal lines and a curve 304 describing the
line width versus focus behavior for vertical lines are shown. As
it is apparent form FIG. 3, the nominal values are different for
both cases. Even more important, however, is the fact that both
curves 302 and 304 show a behavior with different maxima and
slopes, which indicates that best focus conditions are different
for different orientations.
[0036] The behavior is further investigated making reference now to
FIGS. 4A and 4B. FIGS. 4A and 4B both illustrate a proximity
simulation, wherein line elements with a predetermined width having
an adjacent neighboring line are simulated for different distances
between the two lines. This kind of simulation is often a
prerequisite for optical proximity corrections, as explained above.
In these diagrams, for each of the above described orientations a
predetermined focus value is selected during simulation. The
simulation is performed using either a horizontal line (according
to FIG. 4A) or a vertical line (according to FIG. 4B) with a target
line width of 200 nm which has a varying distance to a further
structural element. In both figures, the line width is plotted
against the space to the neighboring further structural
elements.
[0037] In FIG. 4A, a reference line 402 indicating the target value
is depicted. Following this, the reference line 402 shows no
dependency on the distance to the neighboring structural element
and is thus depicted as a flat line at 200 nm line width. The
simulated line width 404, however, shows a strong dependency on the
pitch, i.e. the distance to the neighboring structural feature.
[0038] In a similar way, FIG. 4B depicts a further reference line
408 as a flat line at 200 nm line width. The simulated line width
410 also shows a strong dependency on the distance to the
neighboring structure.
[0039] Simulation results are further illustrated when comparing
the simulated line width 404 for a horizontal line or the simulated
line width 410 for a vertical line with corresponding measured line
width values. In FIG. 4A, measured values 406 for the line width
under similar conditions are depicted. The measured values 406 are
derived from a lithographic projection using lithographic
projection apparatus 100 onto a test substrate. Similar, FIG. 4B
also shows measured values 412 for a vertical line.
[0040] As it can be seen from FIG. 4A, the measured values 406
deviate from the simulated line width 404. In FIG. 4B, there is
also a deviation between measured and simulated images. The
deviation is however less pronounced as compared to FIG. 4A. In
summary, when using dipole illumination and a polarized light
source 104, the simulated and measured data show a strong
orientation-related dependency on beam focus conditions which
results in different best focus settings and furthermore in
different pattern fidelity for differently oriented patterns on a
photo mask.
[0041] This behavior can be attributed by considering aberration
during lithographic simulation. Aberration is usually described
using Zernike coefficients. There, circular wavefront profiles can
be fitted with Zernike polynomials. This leads to a set of Zernike
coefficients that individually represent different types of
aberrations and are linearly independent. Accordingly, individual
aberrations contribute to an overall wavefront and can be isolated
and quantified separately.
[0042] The first Zernike polynomials are equal to the mean value of
the wave front amplitude, describe the deviation of the beam in the
sagittal and tangential direction, describe a parabolic wavefront
shape which results from defocus, attribute to a horizontally or
vertically oriented cylindrical shape, describe flaring in the
horizontal and vertical direction, and are attributed to a third
order spherical aberration.
[0043] Aberration coefficients can be determined by a measurement.
These measurements are usually performed by using wavefront
analyzer system as provided by the ILIAS system from ASML Inc. or
the LITEL test reticle. As a result, Zernike coefficients can be
derived from these measurements which can be forwarded to an
optical model as optical parameters.
[0044] When using an asymmetric illumination mode, e.g., dipole
illumination, the distributed rays form light source 104 yield to
local heating of the lens in lithographic projection apparatus 100.
Local heating is a source for thermal stress which in turn affects
the optical performance and can lead to an increased aberration.
Accordingly, the wavefront measurements can be performed in the
steady state of the illumination optics 104, i.e., after local
heating stresses have reached equilibrium and are constant over
time.
[0045] In FIG. 5A, a characterization of fit accuracy obtained from
the optical model for different focus settings is illustrated.
There, projection of a layout pattern is simulated in both the
horizontal and the vertical direction. The pattern used for
simulation includes a grid of several lines with different sizes
and spacing in order to describe the lithographic projection for
several topological conditions. For both directions, an error
function is calculated which determines the accuracy of the
resulting simulation when compared to the measured value. The fit
accuracy can be determined by statistical functions, i.e.,
calculating an RMS-value or the like.
[0046] For the fit accuracy representation shown in FIG. 5A, no
aberration coefficients have been taken into consideration. The
resulting error function 502 for the vertical direction shows an
optimum setting for the beam focus at a value of approximately 0.03
.mu.m. The resulting error function 504 for the horizontal
direction shows an optimum setting for the beam focus at a value of
approximately -0.07 .mu.m. Following this, there is according to
the fit accuracy representation, no adjustment for the focus
position available which simultaneously optimizes structures in the
horizontal and the vertical direction.
[0047] In FIG. 5B, a similar characterization of fit accuracy
obtained from the optical model for different focus settings is
illustrated. There, aberration coefficients have been taken into
consideration. The resulting error function 506 for the vertical
direction and the resulting error function 508 for the horizontal
direction both show a rather similar optimum setting for the beam
focus at a value between approximately -0.03 .mu.m and -0.01 .mu.m.
Accordingly, an adjustment for the focus position is available
which simultaneously optimizes structures in the horizontal and the
vertical direction.
[0048] With respect to FIG. 6, a flow chart is depicted which shows
individual steps of adjusting an optical parameter of an exposure
apparatus
[0049] In step 600, a photolithographic projection is performed.
The lithographic projection uses the exposure apparatus 100 and
illumination conditions including polarization of light source 104
and/or off-axis illumination. On the photo mask 102 the layout
pattern is provided as the mask pattern 112. As a result of the
lithographic projection, measured pattern data 128 are derived from
the developed resist pattern onto the substrate 122.
[0050] The measured pattern data are provided with different focus
settings of the exposure apparatus 100. During this step, focus
dependent parameters of the printed resist pattern 122 can be
calculated and stored similar as shown in FIGS. 3 and 5. The
measured pattern data are determined in the thermal steady state of
the exposure apparatus 100 in order to achieve stable descriptions
of aberrations caused by lens heating effects.
[0051] In step 602, the optical model is provided. The optical
model is suitable to describe the exposure apparatus 100 under the
selected illumination conditions. This is achieved within the
optical model by including one or more optical parameters, which
are suitable to describe polarization state of light source 104
and/or off-axis illumination.
[0052] Using now in step 604 the optical model together with layout
pattern, a simulated image is created. The simulated image can be
calculated with different focus settings of the exposure apparatus.
During this step, focus dependent parameters of the simulated image
can be calculated. As an example, error functions as described with
respect to FIG. 5 or focus dependency as described with respect to
FIG. 3 can be calculated and stored for further processing.
[0053] In step 606, the optical model is optimized by adjusting the
optical parameter so as to reduce an overall difference between the
measured pattern data and the simulated image. The differences
between the measured pattern data and the simulated image can be
determined along a first and a second direction. As already
explained with respect to FIG. 2, the first and the second
direction can be chosen substantially perpendicular along
horizontally and vertically arranged structural elements of the
layout pattern. An embodiment for optical model optimization could
be, e.g., the introduction and adjustment of aberration parameters,
in order to improve the fit accuracy from a situation as shown in
FIG. 5A to a situation shown in FIG. 5B. Adjustment criterion would
be here the simultaneous achievement of acceptable fit accuracies
for different layout orientations.
[0054] It should be noted that for the step of optimizing the
optical model, measured aberration parameters can be used as
starting value for the optical parameter. However, if the resulting
images are not sufficiently similar, the one or more optical
parameters can be further adapted in an iterative way, in order to
resemble the measured pattern data with higher accuracy.
[0055] The adaptation of the optical parameter of the optical model
can be performed in several ways. It should be noted that either
measured parameters can be used from which Zernike coefficients are
originating. Furthermore, the optical parameter can be extended so
as to not only resemble the true physical aberration but minimize
the differences to the measured pattern. This can be accounted for
by modifying the optical parameter accordingly even so the
parameter appears unphysical from the measurements.
[0056] As a measure for studying the accuracy of the optical model,
best beam focus can be used. To this extent, the optical parameter
is modified to resemble the position of the best beam focus along
two directions, i.e., the horizontal and vertical arrangement of
the structural elements. Optical parameters can furthermore be
modified so as to not only derive best focus positions but minimize
the differences in shape between error functions along different
directions as shown in FIGS. 5A and 5B.
[0057] The result of the procedure is further outlined with respect
to FIGS. 7A and 7B, 8A and 8B, and 9A and 9B. FIGS. 7A and 7B each
illustrate a simulated resist pattern in a top view with respect to
a first aberration parameter. There, the optical model does not
describe aberration. FIG. 7A shows a simulated resist pattern for
the best focus value while FIG. 7B depicts a simulated resist
pattern at a defocus of -90 nm. FIGS. 8A and 8B each illustrate a
simulated resist pattern in a top view with respect to a second
aberration parameter, both for best focus position in FIG. 8A and
at a similar defocus in FIG. 8B as compared to FIG. 7B. FIGS. 9A
and 9B each illustrate a measured resist pattern for both focus
positions.
[0058] From the figures, it can be concluded that the real
situation is quite closely resembled in both simulations for best
focus conditions as differences between FIG. 7A, FIG. 8A and FIG.
9A are rather subtle. In defocus, however, the optical model which
does not properly describe aberrations fails to predict the
measured resist pattern. There are, however, substantially no
differences between FIG. 8B and FIG. 9B which outlines the
performance of the optical model using an optical parameter
suitable to describe aberration in lithographic projection.
[0059] In FIG. 10, a flow diagram is shown in which the concept of
adjusting an optical model is further extended to a model for
optical proximity correction. There, structural elements are
modified, e.g., by modifying their shape or by adding additional
elements, in order to more precisely achieve the desired layout
pattern on a substrate. These modifications include additional
structural elements, hammerheads, serifs and the like. The concept
of optical proximity correction (OPC) is an established procedure
and is well known in the art. In order to derive at a certain
modified layout, the optical projection is simulated using the
above described optical model. In general, optical proximity
correction includes inserting, removing, relocating or modifying
assist features to the layout data. In addition, relocating and
modifying features within the layout pattern can be performed. As a
result, optical proximity correction adapts modified layout data
and layout data to a desired target image.
[0060] In a first step 1000, the optical model is provided which
includes the optical parameter optimized as described with respect
to FIG. 6. In step 1010, the optical projection is simulated using
the optical model. In step 1020, the OPC data are calculated. It
should be noted that for a layout pattern used in manufacturing
different kinds of memory circuits, as, e.g., DRAMs, FeRAM, NROM or
the like, a so-called cell array is present which is located at the
position of the individual memory cells. The cell array comprises
very dense individual elements in order to arrive at high density
memory cells. The cell array is surrounded by periphery structures
which are used to select certain memory cells during operation of
the memory chip. While the cell array comprises a regular pattern,
the periphery structures quite often are represented by different
patterns having line elements both in vertical and horizontal
directions. For lithographic projection, the illumination
conditions are usually selected so as to precisely image the cell
array without any optical proximity correction applied. For the
periphery structures, optical proximity correction is then,
however, even more challenging and requires a very precise optical
model as described above.
[0061] In FIG. 11, a flow diagram is depicted in which the concept
of adjusting an optical model is further extended to a full chip
simulation which can be used to identify critical elements during
lithographic projection.
[0062] In a first step 1100, the optical model is provided which
includes the optical parameter optimized as described with respect
to FIG. 6. In step 1110, the optical projection is simulated using
the optical model. In step 1120, the simulated layout is
inspected.
[0063] With respect to FIG. 12, a system for adjusting an optical
parameter of an exposure apparatus is shown. A measurement device
1200 is provided which can be used to determine a set of measured
pattern data, which were printed on the wafer 122. As a measurement
device, a scanning electron microscope can be used or any other
tool, as a scatterometer suitable to resolve the structures printed
on the wafer can be employed. The measured pattern consists of data
being determined under different focus settings of the exposure
apparatus 100.
[0064] As already described above, the optical model can be used to
describe the exposure apparatus 100. The optical model includes the
at least one optical parameter. As a result from the calculation,
the simulated image is calculated with different focus settings of
the exposure apparatus. A processor 1210, e.g., a computer or any
other device suitable for performing calculations, performs the
adjusting of the optical model in order to reduce the differences
between the measured pattern data and the simulated image.
[0065] Having described embodiments of the invention, it is noted
that modifications and variations can be made by persons skilled in
the art in light of the above teachings. It is therefore to be
understood that changes may be made in the particular embodiments
of the invention disclosed which are within the scope and spirit of
the invention as defined by the appended claims.
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